In certain instruments there is a requirement for data processing with real time coordination of digital information with power line cycle alternating current (AC). Of particular interest is the processing of spectral data from spectral beams interrupted by a chopper wheel in a spectrophotometer. An example of such an apparatus is described in U.S. Pat. No. 4,168,910 (Barnard). Specifically it is necessary to synchronize the digital processing with the timing of a beam as it cyclically passes through the chopper wheel being driven by a synchronous electrical motor. Wheel speed is constant only to the extent that the AC cycles are stable, but may fluctuate if the AC phasing changes.
Such synchronizing for a spectrophotometer with a chopper wheel has been effected by analog circuitry with analog/digital conversion, specifically with a so-called phase lock loop circuit. Time pulses associated with the AC are received by a phase detector which produces a DC signal that is filtered and fed to a voltage controlled oscillator. The oscillator has a feedback loop to the phase detector to maintain synchronization. The oscillations are also frequency divided by 512 or other binary-compatible number to generate synchronized signals during each AC cycle. Such a system is described in "Phase-Locked Loop Design Fundamentals" by Garth Nash, Application Note AN-535, Motorola Inc. (1970).
There are several problems associated with a phase lock loop. These include a sensitivity to variations in component manufacture, and instability or "jitter" during operation. Also the loop circuit takes several cycles to resynchronize to a significant phase shift in the AC. A separate analog/digital conversion circuit is required.
At least one effort has been made to create a digital version of the phase lock loop. This required rather extensive circuitry and actually was a frequency lock loop, therefore lacking real coordination with AC cycle phase edges and requiring several cycles to resynchronize.
Timer chips are, of course, well known with respect to digital clocks. These generally are oscillators based on a quartz crystal. The kilohertz level frequencies are reduced to conventional time units by divider circuits that consecutively divide by two with flip-flops. These timers are not readily synchronizable to other oscillators.
Therefore objects of the present invention are to provide a novel circuit for synchronizing digital signals with alternating current, to provide synchronization of signals to AC with an improved response time to phase changes in the AC, to provide a synchronizing circuit substantially free of instabilities, to provide a relatively simple digital circuit for synchronizing, and to provide a novel circuit for synchronizing processing signals with an AC driven chopper wheel in a spectrophotometer.